3. Contents
• TYPES OF MICROWAVE TUBES
Cavity Klystron
Reflex Klystron
Magnetron tube
Travelling Wave Tube
• MICROWAVE SEMICONDUCTORS
Transferred Electron Devices
Avalanche Transit Time Devices
4. NEED OF MICROWAVE TUBES
• Due to some characteristics the
conventional tubes and transistors are used
at high frequencies mentioned below
• Intra electrode capacitance and lead
inductance effect
• Gain Bandwidth limitation
• Transit time effect
• Skin effect
• Dielectric losses
6. Cavity Klystron
• For the generation and amplification of Microwaves, there is a need of
some special tubes called as Microwave tubes. Of them all,
Klystron is an important one.
• The essential elements of Klystron are electron beams and cavity
resonators. Electron beams are produced from a source and the cavity
klystrons are employed to amplify the signals. A collector is present at
the end to collect the electrons. The whole set up is as shown in the
following figure.
7. • The electrons emitted by the cathode are accelerated towards the
first resonator. The collector at the end is at the same potential as
the resonator. Hence, usually the electrons have a constant speed in
the gap between the cavity resonators.
• Initially, the first cavity resonator is supplied with a weak high
frequency signal, which has to be amplified. The signal will initiate
an electromagnetic field inside the cavity. This signal is passed
through a coaxial cable as shown in the following figure
• Due to this field, the electrons that pass through the cavity
resonator are modulated. On arriving at the second resonator, the
electrons are induced with another EMF at the same frequency.
This field is strong enough to extract a large signal from the second
cavity.
8. Cavity Resonator
• First let us try to understand the constructional details and the working of
a cavity resonator. The following figure indicates the cavity resonator.
• A simple resonant circuit which consists of a capacitor and an inductive
loop can be compared with this cavity resonator. A conductor has free
electrons. If a charge is applied to the capacitor to get it charged to a
voltage of this polarity, many electrons are removed from the upper plate
and introduced into the lower plate.
• The plate that has more electron deposition will be the cathode and the
plate which has lesser number of electrons becomes the anode. The
following figure shows the charge deposition on the capacitor.
9. Working of Klystron
• To understand the modulation of the electron beam, entering the first
cavity, let's consider the electric field. The electric field on the resonator
keeps on changing its direction of the induced field. Depending on this,
the electrons coming out of the electron gun, get their pace controlled.
• As the electrons are negatively charged, they are accelerated if moved
opposite to the direction of the electric field.
• Also, if the electrons move in the same direction of the electric field, they
get decelerated.
• This electric field keeps on changing, therefore the electrons are
accelerated and decelerated depending upon the change of the field.
• The following figure indicates the electron flow when the field is in the
opposite direction.
•
10. • While moving, these electrons enter the field free space called as the drift
space between the resonators with varying speeds, which create electron
bunches. These bunches are created due to the variation in the speed of
travel.
• These bunches enter the second resonator, with a frequency
corresponding to the frequency at which the first resonator oscillates. As
all the cavity resonators are identical, the movement of electrons makes
the second resonator to oscillate. The following figure shows the
formation of electron bunches.
• Amplification of such two-cavity Klystron is low and hence multi-cavity
Klystrons are used.
• With the signal applied in the first cavity, we get weak bunches in the
second cavity. These will set up a field in the third cavity, which produces
more concentrated bunches and so on. Hence, the amplification is larger.
11. APPLICATIONS OF MULTI CAVITY KLYSTRONS
• This type of klystron is mostly used for the
purpose of amplification of microwave
length of frequency
• It means that the high frequencies can be
amplified by multi cavity klystron.
• Which is impossible and not feasible o use
other components for this purpose.
12. Reflex Klystron
• This microwave generator, is a Klystron that
works on reflections and oscillations in a single
cavity, which has a variable frequency.
• Reflex Klystron consists of an electron gun, a
cathode filament, an anode cavity, and an
electrode at the cathode potential.
• It provides low power and has low efficiency.
13. Construction of Reflex Klystron
• The electron gun emits the electron beam, which passes through the gap
in the anode cavity.
• These electrons travel towards the Repeller electrode, which is at high
negative potential.
• Due to the high negative field, the electrons repel back to the anode cavity. In
their return journey, the electrons give more energy to the gap and these
oscillations are sustained.
• The constructional details of this reflex klystron is as shown in the following
• It is assumed that oscillations already exist in the tube and they are sustained
by its operation. The electrons while passing through the anode cavity, gain
some velocity.
14. Operation of Reflex Klystron
• Let us assume that a reference electron er crosses the anode
cavity but has no extra velocity and it repels back after
reaching the Repeller electrode, with the same velocity.
• Another electron, let's say ee which has started earlier than
this reference electron, reaches the Repeller first, but
returns slowly, reaching at the same time as the reference
electron.
• The following figure illustrates this.
15. • We have another electron, the late electron el,
which starts later than both er and ee, however, it
moves with greater velocity while returning back,
reaching at the same time as er and ee.
• Now, these three electrons,
namely er, ee and el reach the gap at the same
time, forming an electron bunch. This travel time
is called as transit time, which should have an
optimum value.
• When the gap voltage is at maximum positive, this
lets the maximum negative electrons to retard.
• The optimum transit time is represented as
• This transit time depends upon the Repeller and
anode voltages.
16. Applications of Reflex Klystron
Reflex Klystron is used in applications where
variable frequency is desirable, such as −
• Radio receivers
• Portable microwave links
• Parametric amplifiers
• Local oscillators of microwave receivers
• As a signal source where variable frequency
is desirable in microwave generators.
17. Magnetron tube
• One microwave tube performs its task so well and so
cost-effectively that it continues to reign supreme in the
competitive realm of consumer electronics: the
magnetron tube.
• This device forms the heart of every microwave oven,
generating several hundred watts of microwave RF
energy used to heat food and beverages, and doing so
under the most grueling conditions for a tube: powered
on and off at random times and for random durations.
• Magnetron tubes are representative of an entirely
different kind of tube than the IOT and klystron.
Whereas the latter tubes use a linear electron beam, the
magnetron directs its electron beam in a circular
pattern by means of a strong magnetic field:
18. • A cross-sectional diagram of a resonant
cavity magnetron. Magnetic field is
perpendicular to the plane of the diagram
19. Travelling Wave Tube
• The traveling wave tube (TWT) is an electron
tube used for amplification at microwave
frequencies – generally identified as
frequencies between 500 MHz and 300 GHz
or to wavelengths measured from 30 cm to 1
mm.
• Travelling wave tubes are broadband
microwave devices which have no cavity
resonators like Klystrons.
• Amplification is done through the prolonged
interaction between an electron beam and
Radio Frequency RF field.
• Power generation capabilities range from watts
to megawatts.
20. Construction of Travelling Wave Tube
• Travelling wave tube is a cylindrical structure which
contains an electron gun from a cathode tube.
• It has anode plates, helix and a collector.
• RF input is sent to one end of the helix and the
output is drawn from the other end of the helix.
• An electron gun focuses an electron beam with the
velocity of light.
• Helix acts as a slow wave structure. Applied RF field
propagated in helix, produces an electric field at the
center of the helix.
• The resultant electric field due to applied RF signal,
travels with the velocity of light multiplied by
the ratio of helix pitch to helix circumference.
21. • The velocity of electron beam, travelling through the
helix, induces energy to the RF waves on the helix.
• The following figure explains the constructional
features of a travelling wave tube.
• Thus, the amplified output is obtained at the output of
TWT. The axial phase velocity Vp is represented as
22. Construction of Travelling Wave Tube
• Where r is the radius of the helix. As the helix provides least
change in Vp phase velocity, it is preferred over other slow wave
structures for TWT.
• In TWT, the electron gun focuses the electron beam, in the
gap between the anode plates, to the helix, which is then
collected at the collector.
• The following figure explains the electrode arrangements in a
travelling wave tube.
• Fig-2
23. Operation of Travelling Wave Tube
• The anode plates, when at zero potential, which means
when the axial electric field is at a node, the electron
beam velocity remains unaffected.
• When the wave on the axial electric field is at positive
antinode, the electron from the electron beam moves in
the opposite direction.
• This electron being accelerated, tries to catch up with
the late electron, which encounters the node of the RF
axial field.
• At the point, where the RF axial field is at negative
antinode, the electron referred earlier, tries to overtake
due to the negative field effect.
• The electrons receive modulated velocity. As a
cumulative result, a second wave is induced in the helix.
• The output becomes larger than the input and results in
amplification.
24. Applications of Travelling Wave Tube
There are many applications of a travelling wave tube.
• TWT is used in microwave receivers as a low noise RF
amplifier.
• TWTs are also used in wide-band communication links
and co-axial cables as repeater amplifiers or
intermediate amplifiers to amplify low signals.
• TWTs have a long tube life, due to which they are used
as power output tubes in communication satellites.
• Continuous wave high power TWTs are used in
Troposcatter links, because of large power and large
bandwidths, to scatter to large distances.
• TWTs are used in high power pulsed radars and ground
based radars.
25. MICROWAVE SEMICONDUCTORS
• Transferred Electron Devices:
The electrons shift from high mobility to low mobility
state under the under influence of strong electric field.
• Ex: Gunn Diode, Tunnel Diode
• Avalanche Transit Time Devices:
When reverse voltage exceeds the junction breakdown
and current flows with slightly increase in voltage.
• Ex: IMPATT Diodes, BARITT Diodes & TRAPATT
Diodes
26. Gunn Effect Devices
• J B Gunn discovered periodic fluctuations of current
passing through the n-type GaAs specimen when the
applied voltage exceeded a certain critical value.
• In these diodes, there are two valleys, L & U valleys in
conduction band and the electron transfer occurs
between them, depending upon the applied electric
field.
• This effect of population inversion from lower L-valley
to upper U-valley is called Transfer Electron
Effect and hence these are called as Transfer Electron
Devices
27. Applications of Gunn Diodes
Gunn diodes are extensively used in the
following devices −
• Radar transmitters
• Transponders in air traffic control
• Industrial telemetry systems
• Power oscillators
• Logic circuits
• Broadband linear amplifier
28. Avalanche Transit Time Devices
• The process of having a delay between voltage
and current, in avalanche together with transit
time, through the material is said to be
Negative resistance. The devices that helps to
make a diode exhibit this property are called
as Avalanche transit time devices.
• The examples of the devices that come under
this category are IMPATT, TRAPATT and
BARITT diodes. Let us take a look at each of
them, in detail.
29. IMPATT Diode
• This is a high-power semiconductor diode, used in
high frequency microwave applications. The full
form IMPATT is IMPact ionization Avalanche
Transit Time diode.
• A voltage gradient when applied to the IMPATT
diode, results in a high current. A normal diode will
eventually breakdown by this. However, IMPATT
diode is developed to withstand all this.
• A high potential gradient is applied to back bias the
diode and hence minority carriers flow across the
junction.
• Application of a RF AC voltage if superimposed on a
high DC voltage, the increased velocity of holes and
electrons results in additional holes and electrons
by thrashing them out of the crystal structure by
Impact ionization.
30. • If the original DC field applied was at the threshold
of developing this situation, then it leads to the
avalanche current multiplication and this process
continues. This can be understood by the following
figure.
• If the original DC field applied was at the threshold
of developing this situation, then it leads to the
avalanche current multiplication and this process
continues.
31. • The multiplication of charges takes place called as avalanche
charge carriers shown in below figure.
• These charges are drifted from the junction results into current
pulse generation.
• The carrier density approximately, reaches its maximum when the
applied electric field has decreased from the peak to the average
value.
• Thus, the ac variation of the injected carrier density lags the ac
voltage by about 90 degrees and this is called as avalanche delay.
32. IMPATT Diode
• Disadvantages
It is noisy as avalanche is a noisy process
Tuning range is not as good as in Gunn diodes
• Applications
Following are the applications of IMPATT diode.
Microwave oscillator
Microwave generators
Modulated output oscillator
Receiver local oscillator
Negative resistance amplifications
Intrusion alarm networks
Police radar [Math Processing
Low power microwave transmitter
FM telecom transmitter
CW Doppler radar transmitter
33. TRAPATT Diode
• The full form of TRAPATT diode is TRApped Plasma
Avalanche Triggered Transit diode. A microwave generator
which operates between hundreds of MHz to GHz.
• These are high peak power diodes usually n+- p-p+ or p+-n-
n+ structures with n-type depletion region, width varying from
2.5 to 1.25 µm. The following figure depicts this.
• The electrons and holes trapped in low field region behind the
zone, are made to fill the depletion region in the diode.
• This is done by a high field avalanche region which propagates
through the diode.
34. • The following figure shows a graph in which AB shows charging, BC
shows plasma formation, DE shows plasma extraction, EF shows
residual extraction, and FG shows charging.
• Let us see what happens at each of the points.
• A: The voltage at point A is not sufficient for the avalanche breakdown
to occur. At A, charge carriers due to thermal generation results in
charging of the diode like a linear capacitance.
• A-B: At this point, the magnitude of the electric field increases. When a
sufficient number of carriers are generated, the electric field is
depressed throughout the depletion region causing the voltage to
decrease from B to C.
35. • C: This charge helps the avalanche to continue and a dense
plasma of electrons and holes is created. The field is further
depressed so as not to let the electrons or holes out of the
depletion layer, and traps the remaining plasma.
• D: The voltage decreases at point D. A long time is required to
clear the plasma as the total plasma charge is large compared to
the charge per unit time in the external current.
• E: At point E, the plasma is removed. Residual charges of holes
and electrons remain each at one end of the deflection layer.
• E to F: The voltage increases as the residual charge is removed.
• F: At point F, all the charge generated internally is removed.
• F to G: The diode charges like a capacitor.
• G: At point G, the diode current comes to zero for half a period.
The voltage remains constant as shown in the graph above. This
state continues until the current comes back on and the cycle
repeats.
36. TRAPATT Diode
• Applications
• There are many applications of this diode.
• Low power Doppler radars
• Local oscillator for radars
• Microwave beacon landing system
• Radio altimeter
• Phased array radar, etc.
37. BARITT Diode
• The full form of BARITT Diode is BARrier Injection
Transit Time diode. These are the latest invention in
this family.
• Though these diodes have long drift regions like
IMPATT diodes, the carrier injection in BARITT diodes
is caused by forward biased junctions, but not from the
plasma of an avalanche region as in them.
• In IMPATT diodes, the carrier injection is quite noisy
due to the impact ionization.
• In BARITT diodes, to avoid the noise, carrier
injection is provided by punch through of the
depletion region.
• The negative resistance in a BARITT diode is obtained
on account of the drift of the injected holes to the
collector end of the diode, made of p-type material.
38. • The following figure shows the constructional details
of a BARITT diode.
• For a m-n-m BARITT diode, Ps-Si Schottky barrier
contacts metals with n-type Si wafer in between.
• A rapid increase in current with applied
voltage above30v is due to the thermionic hole
injection into the semiconductor.
40. Microwave Bench General Measurement
Setup
• This setup is a combination of different
parts which can be observed in detail. The
following figure clearly explains the setup.
41. • Signal Generator
• As the name implies, it generates a microwave signal, in the
order of a few milliwatts. This uses velocity modulation
technique to transfer continuous wave beam into milliwatt
power.
• A Gunn diode oscillator or a Reflex Klystron tube could be an
example for this microwave signal generator.
• Precision Attenuator
• This is the attenuator which selects the desired frequency and
confines the output around 0 to 50db. This is variable and can
be adjusted according to the requirement.
• Variable Attenuator
• This attenuator sets the amount of attenuation. It can be
understood as a fine adjustment of values, where the readings
are checked against the values of Precision Attenuator.
• Isolator
• This removes the signal that is not required to reach the
detector mount. Isolator allows the signal to pass through the
waveguide only in one direction.
42. • Frequency Meter
• This is the device which measures the frequency of
the signal. With this frequency meter, the signal can
be adjusted to its resonance frequency. It also gives
provision to couple the signal to waveguide.
• Crystal Detector
• A crystal detector probe and crystal detector mount
are indicated in the above figure, where the detector
is connected through a probe to the mount. This is
used to demodulate the signals.
• Standing Wave Indicator
• The standing wave voltmeter provides the reading of
standing wave ratio in dB. The waveguide is slotted
by some gap to adjust the clock cycles of the signal.
Signals transmitted by waveguide are forwarded
through BNC cable to VSWR or CRO to measure its
characteristics.
43. PRACTICAL Microwave Bench Setup
• A microwave bench set up in real-time application would look as follows
• Now, let us take a look at the important part of this microwave bench, the slotted line.
44. • Slotted Line
• In a microwave transmission line or waveguide, the
electromagnetic field is considered as the sum of
incident wave from the generator and the reflected
wave to the generator. The reflections indicate a
mismatch or a discontinuity. The magnitude and
phase of the reflected wave depends upon the
amplitude and phase of the reflecting impedance.
• The standing waves obtained are measured to know
the transmission line imperfections which is
necessary to have a knowledge on impedance
mismatch for effective transmission. This slotted
line helps in measuring the standing wave ratio of a
microwave device.
45. • Construction
• The slotted line consists of a slotted section of a
transmission line, where the measurement has to be
done. It has a travelling probe carriage, to let the probe
get connected wherever necessary, and the facility for
attaching and detecting the instrument.
• In a waveguide, a slot is made at the center of the broad
side, axially. A movable probe connected to a crystal
detector is inserted into the slot of the waveguide.
• Operation
• The output of the crystal detector is proportional to the
square of the input voltage applied. The movable probe
permits convenient and accurate measurement at its
position. But, as the probe is moved along, its output is
proportional to the standing wave pattern, which is
formed inside the waveguide. A variable attenuator is
employed here to obtain accurate results.
47. The parts labelled in the above figure indicate
the following.
1. Launcher − Invites the signal.
2. Smaller section of the waveguide.
3. Isolator − Prevents reflections to the source.
4. Rotary variable attenuator − For fine adjustments.
5. Slotted section − To measure the signal.
6. Probe depth adjustment.
7. Tuning adjustments − To obtain accuracy.
8. Crystal detector − Detects the signal.
9. Matched load − Absorbs the power exited.
10. Short circuit − Provision to get replaced by a load.
11. Rotary knob − To adjust while measuring.
12. Vernier gauge − For accurate results.
48. • In order to obtain a low frequency
modulated signal on an oscilloscope, a
slotted line with a tunable detector is
employed. A slotted line carriage with a
tunable detector can be used to measure the
following.
• VSWRVoltageStandingWaveRatio
• Standing wave pattern
• Impedance
• Reflection coefficient
• Return loss
• Frequency of the generator used
49. Tunable Detector
• The tunable detector is a detector mount which is used to
detect the low frequency square wave modulated
microwave signals. The following figure gives an idea of a
tunable detector mount. It is terminated at the end and
has an opening at the other end just as the above one.
50. To provide a match between the Microwave
transmission system and the detector mount, a
tunable stub is often used. There are three
different types of tunable stubs.
• Tunable waveguide detector
• Tunable co-axial detector
• Tunable probe detector
Also, there are fixed stubs like −
• Fixed broad band tuned probe
• Fixed waveguide matched detector mount
The detector mount is the final stage on a
Microwave bench which is terminated at the
end.
